Embodiments of the present disclosure are related to wind turbines, and more particularly to a system and method for controlling wind turbines.
In wind turbine control systems, cyclic pitch control also known as Rotor Imbalance Control (RIC) is used to mitigate rotor imbalance loads in a yaw axis and a nodding axis that arise due to sampling of a non-homogenous wind field by the wind turbine blades. Cyclic pitching of the blades at the static (0P), rotational (1P), and multiple (for example, 2P) frequencies facilitates reducing the energy in the 0P, 1P, and 2P frequencies in the yaw axis and nodding axis imbalance loads. The activation of this RIC is conditioned on a current power output of the wind turbine, which is correlated to the expected yaw axis and nodding axis imbalance loads at each potential power output level. As the power output transitions from a low value, for example, about 65% rated power to a higher value, for example, approximately 80% rated power, RIC transits from zero activation to full activation. This activation scheme assumes a certain monotonic relationship between the rotor imbalance loads and power output of the turbine. However, there are scenarios where this relationship is not preserved and the turbine might experience high rotor imbalance loads even at low power outputs. Such situations can be conditions of high wind shear, wind misalignment at low/medium wind speeds and extreme turbulence. In such scenarios, even though the RIC subsystem could help mitigate the rotor imbalance loads, it stays deactivated due to low power output. One possible approach to alleviate such a situation is to lower the threshold on power to activate the RIC subsystem. However, that imposes penalties on annual energy production (AEP) by pitching the blades when not needed and by adding to pitch control duty cycle.